It is always a good idea to know your competition and one aspect is to know which other websites and domains they own to get an idea of their capability to push a website. This can be done by checking other websites that are hosted at the same IP of a website that you want to check out. This does not always bring the desired result though and is only useful if the competitor uses his own server. Additional IPs and servers make it more difficulty but it is still a good way to learn a few things about the competitor.

A script that can be used is Reverse IP Domain Check, there are others that you can find using Google or other search engines that reveal the same information. Just enter a domain name or IP in the form and wait a few seconds until the results are revealed. All domains hosted on the same IP address are revealed.

You could now check out a few of the domains to see if they belong to the same company which is likely if they link to each other or if the copyright / contact information are identical. A look at those websites will give you valuable information about the competitor. If he has a lot of well visited websites, high Pageranks, unique advertises you can almost be sure that you are up for a fight.

If your competitor is hosted on a shared host you can normally assume that an individual is running the website. It does not mean that the individual is not capable of SEO but it normally means that you are not up against a company of some sort that uses it marketing power effectively to push their website to the top.